The invention relates to a method and a device for controlling the negative-pressure level of a lateral pulling device for laterally aligning sheets by a pulling bar subjectible to the action of suction air at different pressure levels dependent upon the pulling path.
Such a device has become known heretofore from the published German Patent Document DE 35 21 691 A1 which discloses a rotary valve for controlling suction air for a pulling bar for laterally aligning sheets. A control groove of the rotary valve is thereby disposed so that an inlet opening for suction-air feed is already closed when the control groove reaches an opening for fresh-air feed. This measure makes it necessary to provide an additional solenoid valve which is switched at high frequency, with the result that fresh air or compressed air is fed to the control groove. This measure continuously decreases the retaining force between the pulling bar and a sheet.
It is accordingly an object of the invention to provide a method and a relatively simple device for ensuring that the sheets which are to be aligned are transported reliably against a lateral stop and strike against the lateral stop with a low retaining force and, irrespective of the pulling path, virtually at the same speed.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a method of controlling negative-pressure level of a lateral pulling device for laterally aligning sheets through the intermediary of a rotary valve, which comprises having, during an accelerating phase of the lateral pulling device, a high-level negative pressure at a suction pull bar of the lateral pulling device; upon reaching a maximum and constant speed of the suction pull bar, a medium-level negative pressure at the suction pull bar; and, after reaching a desired pulling path of the suction pull bar, up to an end position of the suction pull bar, a low-level negative pressure and no negative pressure, respectively, at the suction pull bar.
In accordance with another mode, the method of the invention comprises having, during a rearward movement of the suction pull bar, no negative pressure at the suction pull bar.
In accordance with a further aspect of the invention, there is provided a device for controlling negative-pressure level of a lateral pulling device for laterally aligning sheets which are fed to a sheet-processing machine, including a suction pull bar movable in time with the sheet-processing machine and subjectible to action of suction air at different pressure levels, and a rotary valve having a rotary part formed with a control groove for controlling the suction air, comprising structure defining a connecting opening to the negative-pressure source, a connecting opening to the suction pull bar and an aerating opening, the control groove being of such length as to connect said openings to one another at least once during an operating cycle.
In accordance with another feature of the invention, the control groove is formed with an end having a pass-through cross section that is reduced in size relative to the rest of the control groove.
In accordance with a further feature of the invention, the openings are arranged within an angle smaller than 180xc2x0.
In accordance with an added feature of the invention, the device includes a suction channel connecting the suction pull bar to the rotary valve, the suction channel having a width, at an opening to the moving suction pull bar, that is wider by a multiple than the diameter of the suction channel.
In accordance with an additional feature of the invention, the rotary valve is drivable via a mechanical coupling with the sheet-processing machine.
In accordance with a concomitant feature of the invention, the rotary valve is drivable by an adjusted electric motor.
It is an advantage of the invention that the sheets which are to be aligned laterally are aligned gently against the lateral stops, thereby avoiding damage to the lateral sheet edge. At the same time, a rebounding of the sheet from the lateral stop is also avoided, with the result that the sheet can be fed very precisely to the sheet-processing machine.
A high retaining force between the sheet and the pull bar is advantageously produced during an accelerating phase of the pulling device for the lateral alignment of the sheet. After a short accelerating path, the pressure is lowered extremely quickly because only a small retaining force is required for transporting the sheets at constant speed. Upon reaching the end of the pulling path, the opening for the suction-air feed is fully covered and the pull bar is connected, via the control groove, to a bypass opening to ambient air, and as a result there is no longer any retaining force present between the pull bar and the sheet. This state is also maintained when the pull bar executes its rearward movement.
It is also extremely advantageous that the pulling path can vary within a wide range (approximately 2 to 10 mm), the conditions for arrival at the stop (speed and retaining force) being kept constant. This results in a robust sheet travel.
In an advantageous configuration of the subject matter of the invention, the control groove is formed with such a length that all the openings of the rotary valve, i.e., to the suction-air source, to the suction pull bar and to the bypass, are connected to one another at least once during an operating cycle. In a further advantageous configuration, the openings are arranged over an angle range smaller than 180xc2x0.
In an advantageous development of the subject matter of the invention, provision is made for the control groove to be provided, in the end or trailing region thereof, with a smaller pass-through cross section than in the front or leading region thereof. This measure results in maintaining the energy consumption, brought about by the pressure drop at the bypass valve, at a low level.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a negative-pressure control for a lateral pulling device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: